Microlithographic systems are employed to allow creation of micron and sub-micron width patterns on semiconductor wafers during the processing of semiconductor devices. In such systems, particularly X-ray and optical lithography systems, control of wafer flatness is critical in order to achieve the desired widths. Wafer flatness of one micron or better is desirable for sub-micron lithography.
Conventional semiconductor wafers tend to be relatively flexible because of their large diameter to thickness ratios (approximately 150:1). This necessitates the use of highly specialized polishing techniques in order to achieve wafer flatness of 10 microns or better. Even with the use of such techniques, flatness deviations during high temperature processing are common.
It is known in the prior art to use extremely flat rigid vacuum chucks to hold down and flatten the wafer during lithographic exposure of the wafer. However, it is difficult to polish the chucks to the necessary flatness and to create wafers having the necessary uniformity of thickness to achieve the desired wafer flatness.
In accordance with the illustrated preferred embodiment of the present invention, a deformable vacuum chuck is mounted on a rigid base by a number of piezoelectric stacks (PZTs). An electric potential may be applied across selected PZTs in order to deform the chuck to compensate for flatness deviations in the wafer.